I.

The Various Physiological and Psychological Rhythms--Menstruation--The Alleged Influence of the Moon--Frequent Suppression of Menstruation among Primitive Races--Mittelschmerz--Possible Tendency to a Future Intermenstrual Cycle--Menstruation among Animals--Menstruating Monkeys and Apes--What is Menstruation--Its Primary Cause Still Obscure--The Relation of Menstruation to Ovulation--The Occasional Absence of Menstruation in Health--The Relation of Menstruation to "Heat"--The Prohibition of Intercourse during Menstruation--The Predominance of Sexual Excitement at and around the Menstrual Period--Its Absence during the Period Frequently Apparent only.